Glass encased resistor and method of making same



R. w. BRoNsoN Er AL 3,012,214

Dec. 5, 1961 GLASS ENCASED RESISTOR AND METHOD OF MAKING SAME Filed Aug. 7. 1959 2 Sheets-Sheet 1 ZILIIII Dec. 5, 1961 R. w. BRoNsoN Erm. 3,012,214

GLASS ENCASED RESISTOR AND METHOD 0F' MAKING SAME Filed Aug. '7, 1959 2 Sheets-Sheet 2 'o CARBON 2 3 CARBON CAR BON (8 n.2 wn b 0n. www VBK. im r em lD .r OO RN l l l United States Patent O 'ice 3,012,214 GLASS ENCASED RESISTOR AND METHOD F MAKING SAME Robert W. Bronson, Dallas, and Norman D. Korbitz,

Richardson, Tex., assignors to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Aug. 7, 1959, Ser. No. 832,301 8 Claims. (Cl. 338-237) with the body, a glass bead at each end of the body, and

an airtight glass housing surrounding the body and the beads; and to provide a method `of fabricating such a v resistor.

A still further object is to provide a high speed, quantity production method for producing a glass encased resistor by using precut glass tubes of the exact length to eliminate waste.

These and other objects and advantages will be apparent from an examination of the following specification and drawing in which:

FIG. l represents a block diagram of the process of making resistors according to this invention;

FIG. 2 is a detail side view of a. typical resistor blank used in this invention;

FIG. 3 is an end view of the device of FIG. 2 after a surface of carbon has been deposited thereon;

FIG. 4 is a side view of the device of FIG. 3 after the step of applying silver to the ends thereof;

FIG. 5 is a side View of the device of FIG. 4 after a helical groove has been cut in the carbon shell of the body;

FIG. 6 is an exploded view partly in cross-section showing'one step in the method of fabricating the resistor of this invention;

FIG. 7 is a schematic view of the RF heating stage in assembling the leads to the blank;

FIG. 8 is a side view of the completed resistor body assembly being inserted in the glass tube. blank;

FIG. 9 is a vertical View, partly in cross-section, of the assembly of the resistor body assembly and the glass tube during one step in this method;

FIG. l0 is an outline view of the method step immediately following the step of FIG. 8 in the production of the resistor shown in its entirety in FIG. 10;

FIG. 11 is an elevational view of the finished resistor of this invention; and

FIG. 12 is an end View of the device of FIG. 1l.

The complete resistor assembly, asshown at 2 in FIG. 11, is the product of this invention and is produced by the method of this invention, as hereinafter disclosed. This resistor assembly 2 is seen to include a pair of axially extending leads 3 having a glass bead 4' near the end of the lead which is attached to the resistor body 12. During fabrication, each bead 4 becomes fused to both the lead 3 and the glass tube 6 to provide a hermetically sealed glass encapsulated resistor assembly.

The principal steps in the process of thisxinvention are depicted in block diagram form in F'IG. 1, and the complete process is thus disclosed byreference to the block 3,012,214 Patented Dec. 5, 1961 diagram in conjunction with the other figures of the drawing.

'In FIG. 1, it is seen that the first step in the process of this invention is to join a short length of copper wire 3A to a much shorter piece of Kovar wire 3B by butt welding at 7 to form a lead 3 for subsequent assembly into each end of the assembly 2. Each of these leads 3 is then degassed and degreased in a hydrogen furnace at 900 C.l for 15 minutes. A glass bead 4 made from a borosilicate glass known in the trade as Corning Glass Works 7052 is next slipped over the lead 3 to a position at or over the weld junction 7 and heated and fused thereto to form a lead assembly 3 fas shown in FIG. 6.

This lead assembly 3 is next cleaned in a nitric acid wash. While the above process steps are being performed to produce the lead assemblies 3', a series of process steps are also being performed on the resistor blank 5 so that at 'the conclusion of each of these parallel operations the leads and body may be joined prior to encapsulation. The resistor blank 5 is a solid cylindrical member of aluminum oxide into each end of which has been drilled a short length axial hole indicated at 10 in FIG. 2. Since the aluminum oxide has insulating properties, some conducting or semiconducting material must be added to provide the desired degree of resistance to the assembly. This is accomplished by vapor depositing a surface coating of carbon -to the desired thickness (the estimated thickness range being from .0l to 5 104 cm.) on the cylindrical periphery of the resistor blank 5 by the decomposition of methane gas in contact with the blank in an enclosed furnace to form the thin surface shell 8 of carbon, :as shown in FIG. 3. The next step in the process of preparing the blank 5 is to paint a layer 9 of silver onto each end and into the counterbore 10 as shown in FIG. 4 and then bake in the silver to insure uniform coverage. At this point, the prepared blank is placed in a groove cutting machine and the helical groove 11 is cut completely `through the carbon shell 8 to provide an increased resistance path having a Vspecilic ohmic value between the silvered ends 9 of the blank to thus form the unit identified .as resistor body 12 as shown in FIG. 5.

Having completed the two separate parallel process paths shown in FIG. l, the ltwo subassemblies 3. and 12 are then assembled by first dipping the Kovar end 3B of the lead assemblies 3 into silver paste and then inserting this end into the counterbore 10 over a small tin ball 10a to Iform the complete body assembly 13 as shown in FIGS. 6 and 7. v

The next process step is to cure the body assembly 13 and simultaneously rigidly attach the leads in place by first baking the assembly 13 in an air oven at 200 for two hours and then placing each end in a radio frequency induction heating coil C to thoroughly solder the leads to the body by a complete soldering ofthe tin to bothV the silver surface and the silver paste covered Kovar leads, and to decompose any organic material present (FIG. 7). The body assembly 13 is now ready to be encapsulated within ,a glass tube 6 and the latter is slipped over one end as shown in FIG. 8. The now enclosed but unsealed assembly is passed by a burner 15 to preheat and start the'closing operation. The flame of burner 15 includes suflicient pressure to force the upper end 14 of the tube 6 in a direction to close about the bead 4 and the tube 6 and body assembly 13 are simultaneously rotated While the flame is applied so that a smooth rounded closure tip as shown in FIG. 9 will result. During this heating and closing operation, nitrogen is being circulated in and through the shell 6 so that an inert atmosphere inside the shell 6 will result. The

flow of nitrogen (N2) through the shell 6 must be carefully controlled becau-se if the ratio of oxygen to nitrogen within the shell becomes too large, they oxygen and carbon may ignite and this will change the resistance value of thel finished unit; also if the nitrogen pressure becomes too large it will prevent proper closing of the sealed end of the tube 6. The nitrogen flow during the sealing operation also accomplishes another desired result in that it leaves a bright finish on the silver plated surface. A cold air blast nozzle 16 is trained upon the periphery of the shell 6 ata point just below that at which the ame from burner 16 is applied and this combination of simultaneous heat forming and cooling will prevent the formation `of air bubbles in the molten portion of the glass as shown in FIG. 10. As the topopen end approaches a complete closure of the open end, the internalatmosphere is cooled by the air blast and the internal pressure is thus reduced until Vthe seal is completedand the combination of the llame pressure and the external atmospheric pressure will form aV smooth closure about the bead 4 Vand wire 3 and also the tendency for air bubbles' to form in the glass will be reduced since the bubbles will pass on into the reduced internal pressure area before the pressure can be built up through the restricted passage between the body 13 and the shell 6. A view of the sealed end of the resistor is shown in FIG. 12. After one end is completely sealed by this operation, which takes approximately three seconds to complete, the'assembly is inverted andthe other end is then fused around its bead and wire in the same manner as that described above for the first end to forma complete resistor assembly. At this stage, nitrogen is no longer being circulated through the assembly, however, due to the short Itime involved and due to the confined passages around the body 12, there will be a concentrated nitrogen atmosphere remaining within the shell which will expand during the heat sealing operation and provide a substantially neutral atmospherewithin the sealed shell 6.

It will be recognized that a resistor assembly manuvfactured by thev process of this invention will have the following advantages due to the .method conditions under -whichit was made:

(a) The use of glass beads, together with theapplica-V tion of cold air, prevent kheat from the flame of burner l v1'5 and fromthe molten area of the glass tube from dam- L aging the ends of the resistor.

(b) The glass-to-glass seal insures an airtight enclosure. Y

(c) The oxidized Kovar wire provides a proper bond f for the lglass bead whereas a copper Wire would not.

(d) The Kovar wire is receptive to induction heating so that the heating occursinternaliy where needed toV fuse the bead to the ywire and thus the fusing can be accomplished with less heat ,than if applied externally to the bead.

r(e.) The formed portion of the glass shell will not pickup air bubbles due to the reduced internal pressure created by thecoldair jet. i

' of said leads, depositing conductive material on one external surfaceof a non-conductive blank, adding a solderable conductive material to other surfaces of said blank and in electrical continuity with said first named conductive surface, placing said leads into engagement with said solderable material and into a recess adjacent each of said otherl surfaces and heating said inserted leads to bond the Vleads to saidblank, and encasing said blank, beads, and a portionv of said leads in a glass shell by fusing the ends of said shell to said beads.

2. A method of making a glass encased resistor assembly asin claim l wherein said bonding step is carried kout by inductively heating the inserted portion of said leads through the surrounding blank.

3. A method of making a glass encased electrical circuit component assembly comprising: fabricating acomponent body having leads projecting therefrom and having a glass bead at juncture of each lead, enclosing said body and beads and a gas in a glass shell, sealing the ends of vsaid shell by fusion with the glass beads, and

applying a jet Vof cold air to the external surface of the shell near the end being sealed at such a rate as to prevent an undesirable temperature rise of the gas inside the shell during the sealing operation. n

4. A method of making a glass encased electrical circuit component assembly comprising: placing a component body having leads projecting therefrom and having a glass bead at the juncture of each lead, enclosing said body and beads and a gas in a glass shell, passing nitrogen through said shell while sealingV one end of said shell to one. of said beads, and then sealing -said other end of said shell to said other bead while a substantial portion of said nitrogen remains within said shell, and applying a jet of cold air to the external'surface of said shell near each end as it is being sealed at vsuch a rate as to prevent an undesirable temperaturerise of the gas inside the shell during the sealing operation. t

5. A method of making a glass encased electrical circuit component assembly comprising: placing acomy face of said shell lnear each end as it is being sealed at (j) The. use of cut length glass tubesv will provide a i more economical construction than afforded by other methods using continuous tube processes. l Wl'iile` this method contemplatesthe use of Kovar material lead wire section (Kovar being the commercial t namev of a particular iron-nickel-cobalt alloy) together with a hard` glassv bead, a. similar satisfactory result may be obtained by usinga Durnet material lead wire and aV soft glass, Dumet being the trade name for a nickeliron alloy having a copper, coating thereon. A suitable Y,

soft glass for use withV Durnet wire has been found to be that known in the trade as Corning Glass CojG-12.

Although certain specific embodiments of the invention have been shown andydescribed, it is obvious that many modifications thereof are possible. The invention, therefore, is not'to be restricted except insofar as is necessitated by the prior Yart and by the scope of the appended claims.

such a rate as `to prevent an undesirable temperature rise of the gas inside theshell dix-ring the sealing operation, and rotating said shell and body during both sealing operations.

'6. A method rof making a glass encased resistor assembly comprising: joining separate material sections of `lead wire materials together to: form complete" leads in which only a portion thereof is both fusible to glass and receptive to induction heating, degassing said leads in a v hydrogen atmosphere, fusing -a glass bead to the fusible portion of said leads, depositing conductive material on one external surface-of a non-conductive blank, adding a solderable conductive material toother surfaces of said blank and in electrical continuity with said firstnamed conductive surface, coating the ends of said leads with silver paste, placing said coated ends of said leads into engagement with said solderable material and into a recess adjacent each of said other surfaces to form a resistor body unit and curing said body unit at a low temperature, and inductively heating said inserted lead ends to solder the leads to said blank, placing a glass shell ventirely over the body unit and partly over said leads, rotating said shell and body unit and passing nitrogen therethrough, directing a pressure llame against the upper end of said shell in a direction to cause said glass end to form toward the lead and bead while in its molten state, and applying a jet of cold air to the external surface of said shell at the instant the glass becomes pliable to assist the closing action of the flame, and then llame sealing the other end of said shell to form a complete resistor assembly.

7. A method of making a glass encased resistor assembly comprising: buttwelding separate sections of Kovar and copper wire together to form lead wires, degassing said lead wires in a hydrogen furnace at approximately 900 C. for about fteen minutes, fusing a glass bead over the fusible portion of said leads and overlapping said butt-Weld, depositing a thin layer of carbon on the peripherial surface of a non-conductive cylindrical blank having axial counterbores, adding a layer of silver to each end ot said blank including said counterbores and in electrical continuity with said carbon surface, cutting a helical groove through the surface layer of said carbon to form an increased resistance path on said coated blank, dipping the Kovar ends of said leads in silver paste and inserting said dipped ends into said counterbores to form a resistor body unit and curing said body unit in an air oven at approximately 200 C. for about two hours, placing each end of said body unit in a radio frequency induction heating coil and soldering said leads into firm attachment with said blank, placing a glass tube over said body unit and rotating said tube and body unit and passing nitrogen therethrough, directing a pressurized flame against the upper end of said tube in a direction to form said glass toward said lead and fusing said end to said glass bead, and applying a blast of cold air to the external surface of said shell at the instant the glass becomes pliable to assist the closing action of the flame, and sealing the other end of said shell within a time when a substantial portion of the nitrogen passed through the shell during the rst sealing step remains entrapped therein.

8. A hermetically sealed electric component assembly, comprising: an electrical element, leads extending from opposite surfaces of said element, a fusible glass bead abutting each said surface and fused to each said lead, a fusible glass shell surrounding said element, said beads and a portion of said leads, said shell being hermetically sealed to both said bead and said lead and being spaced from said element by the presence of said beads.

References Cited in the file of this patent UNITED STATES PATENTS 1,547,701 Weeks July 28, 1925 2,043,196 Finlayson June 2, 1936 2,416,599 Victoreen Feb. 25, 1947 2,445,073 Marette July `13, 1948 2,940,161 Elarde June 14, 1960 

